It has long been known that paper pulp can be bleached using chlorine as the primary oxidant. However, the bleaching of paper pulp with chlorine, while still widely practiced, has come in disfavor due to the discovery of the presence of 2,3,7,8 TCDD (Dioxin) and 2,3,7,8, TCDF (Furan) in the waste products of chlorine bleached pulp. These two undesired by-products are believed to be cancer causing chemicals when present in detectable levels. Attempts to destroy these particulates, even with supercritical wet oxidation, may not be adequate to reduce the concentration of these particulates to acceptable levels. As a result, efforts have been made to replace chlorine bleaching with oxygen bleaching.
Known methods of bleaching with oxygen have not provided satisfactory results due to the inefficient batch nature of processing as well as the intensive mechanical nature of system pumps which tend to damage the pulp. Additionally, these systems have problems with product inconsistency, tank short circuiting, and difficulties relating to the doubly heterogeneous mixtures of pulp, water, and oxygen, the end product being either under oxidized or over oxidized. Thus, because of the damage done to the cellulose and hemi-cellulose envelope due to the intensive mechanical nature of, and the inconsistent oxidation by, these processes, such have not gained significant acceptance.
Recently, many pumped and batch tank processes have been replaced by hydraulic column or gravity pressure vessel devices. These methods are continuous in nature but existing art has not addressed the needs of paper pulp bleaching. Typical of these processes are disclosed in Land et al. U.S. Pat. No. 3,464,885 and Titmas U.S. Pat. No. 3,853,759, both of which teach continuous gravity pressure vessels for processing mixtures of fluids and solids in suspension. Both, however, do not teach more than the introduction of heat to induce pyrolytic reaction and neither disclose a method or apparatus which can provide for the temperature controls and enhanced mass transfer of oxygen into solution necessary for the surgical control of oxidation bleaching.
One gravity pressure vessel patent, McGrew U.S. Pat. No. 4,272,383, does provide a fixed general cooling capability using an oil heat exchanger, but it does not provide cooling at variable positions within the vessel adequate for the needs of paper bleaching. Paper bleaching is optimized between 200.degree. F. and 300.degree. F. and such control capability could not exist in the McGrew apparatus.
Moreover, the McGrew, Land, and Titmas patents all involve control throttles that would mechanically damage the fragile cellulose fiber and all teach the use of pumping devices for start up and circulation. In addition, the use of steam heat, as contemplated by the Land and Titmas patents at the high pressures desired for the mass transfer of oxygen into solution, would cause the mix to be so hot so as to seriously damage the cellulose fiber and its hemi-cellulose sheath. The McGrew patent discloses the optimization of heating to the saturation temperature of steam and water, which, like Land and Titmas, would destroy the fragile cellulose composition thereby forming carbohydrates, starches and undesirable byproducts.
Thus, the unique needs of the bleaching of paper pulp go unresolved and the industry struggles for an environmentally sound and efficient means to bleach pulp.